1. Field of the Invention
The present invention relates to an image forming apparatus including a drive transmission device for driving a photosensitive drum and the like.
2. Description of the Related Art
Hitherto, as an electrophotographic image forming apparatus, a tandem type image forming apparatus for forming full color images has been provided. The tandem type image forming apparatus includes multiple image forming portions. Thus, depending on machine accuracy or the like, speed fluctuation or the like of multiple photosensitive drums and a transfer belt may occur unequally in respective colors, and images may be shifted when superimposing each other. In this way, color misregistration may occur.
There are two types of color misregistration, that is, regular color misregistration and irregular color misregistration. The regular color misregistration is caused by, for example, a failure in positioning during assembly of laser scanners of the respective colors. The irregular color misregistration is caused by, for example, rotational speed fluctuations of the photosensitive drums, drive rollers of the transfer belt, and the like.
In order to suppress the irregular color misregistration, frequency variation components of a drive system for the multiple photosensitive drums and the transfer belt need to be prevented from appearing in images. In Japanese Patent Application Laid-Open No. S63-11967, there has been proposed a technology for reducing image deterioration due to the frequency variation components. In the technology described in Japanese Patent Application Laid-Open No. S63-11967, the multiple photosensitive drums are driven by a common drive source, and arranged in a manner that a time interval in which the transfer belt passes adjacent transfer positions is equal to an integral multiple of a drive fluctuation cycle of the drive source.
Specifically, the photosensitive drums are arranged in a manner that a time interval from an exposure timing of a first photosensitive drum located on an upstream side in a transfer member moving direction to an exposure timing of a second photosensitive drum located adjacent to the first photosensitive drum and on a downstream side in the transfer member moving direction is substantially equal to an integral multiple of a drive fluctuation cycle of each idler gear. In addition, the photosensitive drums are arranged in a manner that a time interval from a transfer timing of the first photosensitive drum to a transfer timing of the second photosensitive drum is also substantially equal to the integral multiple of the drive fluctuation cycle of each of the idler gears.
This technology has a problem in that, in a case where the multiple photosensitive drums respectively have different meshing angles of the idler gears for transmitting a drive force to the photosensitive drums, rotational speeds of the photosensitive drums vary from each other at the same timing when transferring or exposing, resulting in the color misregistration.
FIG. 7 is a sectional view of a configuration of primary transfer rollers 12, photosensitive drums 1, and a drive transmission device 560 of a related art image forming apparatus. With reference to FIG. 7, the principle of occurrence of the color misregistration is described. In the color image forming apparatus illustrated in FIG. 7, a processing speed is set to 123.99 mm/s, a distance L between two colors is set to 53.74 mm, and a diameter of each of the photosensitive drums 1 (1Y, 1M, 1C, and 1K) is set to 24 mm. Drive gears 18 (18Y, 18M, 18C, and 18K) rotate the photosensitive drums 1 (1Y, 1M, 1C, and 1K) at a peripheral speed that is equal to the processing speed of 123.99 mm/s. Therefore, the number of rotations of each of the photosensitive drums 1 (1Y, 1M, 1C, and 1K) is expressed by 123.99/(π×24)≈1.644 (rps).
The number of teeth of each of the drive gears 18 (18Y, 18M, 18C, and 18K) is set to 94, and the number of teeth of a pinion of each branch gear 30 is set to 67. Thus, the number of rotations of each of the branch gears is expressed by 94/67×1.644 (rps)≈2.307 (rps). Therefore, a cycle G of each of the branch gears 30 is expressed by 1/2.307≈0.434 (sec).
The distance L between the two colors is set to 53.74 mm, and the processing speed is set to 123.99 mm/s. Therefore, a time interval between the two colors is expressed by L/v=53.74/123.99≈0.433 (sec). In this color image forming apparatus, the cycle G is set so as to satisfy G≈L/v. In other words, with setting of L/v=n×G (n: integer number), rotational speed fluctuations of each of the branch gears 30 are canceled in the distance L between the two colors. In this way, the color misregistration is reduced.
However, the two branch gears 30a and 30b, which are commonly and identically molded, have meshing angles respectively set between idler gears 31a and 31b and the drive gears 18Y and 18C so as to be different angles of 148.5° and 246.4°. In addition, the commonly and identically molded idler gears 31 (31a and 31b) have meshing angles respectively set between a transmission gear 32 and the branch gears 30 so as to be different angles of 125.3° and 142.1°.
Therefore, between the drive gears 18Y and 18C (and drive gears 18M and 18K) to which drive is respectively transmitted from the transmission gear 32, the rotational speed fluctuations of each of the branch gears 30 and rotational speed fluctuations of each of the idler gears 31 cannot be matched with each other. As a result, the color misregistration occurs between the photosensitive drums 1Y and 1C (and 1M and 1K) of the two colors.
FIG. 8A is a graph illustrating the rotational speed fluctuations of the branch gear 30a arranged on an upstream side in a sheet moving direction. FIG. 8B is a graph illustrating the rotational speed fluctuations of the branch gear 30b arranged on a downstream side in the sheet moving direction. FIGS. 9A and 9B are graphs each illustrating comparison between the rotational speed fluctuations of each of the branch gear 30a and the branch gear 30b. With reference to FIGS. 8A, 8B, 9A, and 9B, the mechanism of occurrence of the color misregistration is described.
Rotational speed fluctuations 34 (34a and 34b) at meshing positions between the branch gears 30 (30a and 30b) and the idler gears 31 (31a and 31b), in other words, at the time of drive input are indicated by broken lines. Rotational speed fluctuations 35 (35a and 35b) at meshing positions between the branch gears 30 (30a and 30b) and the drive gears 18Y and 18C on the upstream side in the transfer direction, in other words, at the time of drive output are indicated by dashed lines. It has been generally known that the rotational speed fluctuations of gears are caused as shown in FIGS. 8A, 8B, 9A, and 9B in one rotation cycle due to, for example, decentering of the gears.
In FIG. 8A, in a case where drive is input from the idler gear 31a when rotating the branch gear 30a on the upstream side in the transfer direction, which transmits drive to the drive gears 18Y and 18M on the upstream side in the transfer direction, at a maximum rotational speed, the rotational speed at a position rotated by 180° from the meshing position between the branch gear 30a and the idler gear 31a is a maximum rotational speed at the time of drive output. Thus, in the branch gear 30a that is rotated by 148.5° to mesh with the drive gear 18Y on the upstream side in the transfer direction, the peak of the rotational speed in the rotational speed fluctuation 35a at the time of drive output leads by a time period corresponding to 180−148.5=31.5(°) with respect to that of the rotational speed fluctuation 34a at the time of drive input.
Thus, a net rotational speed fluctuation of the branch gear 30a on the upstream side in the transfer direction is equal to a sum of the rotational speed fluctuation 34a at the time of drive input and the rotational speed fluctuation 35a at the time of drive output. As a result, a rotational speed fluctuation 36a as indicated by a solid line in FIG. 8A is obtained.
Similarly, in FIG. 8B, in a case where the branch gear 30b on the downstream side in the transfer direction, which transmits drive to the drive gears 18C and 18K on the downstream side in the transfer direction, meshes with the idler gear 31b when rotating the branch gear 30b at a maximum rotational speed, the branch gear 30b meshes with the drive gear 18C after being rotated by 246.4°. Thus, the peak of the rotational speed in the rotational speed fluctuation 35b at the time of drive output lags by a time period corresponding to 246.4−180=66.4(°) with respect to that of the rotational speed fluctuation 34b at the time of drive input. As a result, a net rotational speed fluctuation of the branch gear 30b is obtained as a rotational speed fluctuation 36b indicated by a solid line in FIG. 8B.
In a state of FIG. 9A, a meshing phase of the branch gear 30a on the upstream side in the transfer direction and a meshing phase of the branch gear 30b on the downstream side in the transfer direction are matched with each other so as to match the rotational speed fluctuations with each other. Even in this case, as shown in FIG. 9B, the rotational speed fluctuations 36a and 36b of the branch gears 30a and 30b cannot be perfectly matched with each other. As a result, color misregistration occurs by a distance corresponding to a difference Δ in rotational speed.
In addition, the idler gears 31 (31a and 31b) also have different meshing angles respectively set between the transmission gear 32 and the branch gears 30a and 30b. In FIG. 7, the meshing angle of the idler gear 31a on the upstream side in the transfer direction is 125.3°, and the meshing angle of the idler gear 31b on the downstream side in the transfer direction is 142.1°. In this case, color misregistration occurs in a manner similar to that of the two branch gears 30a and 30b. 